In a vehicular ad hoc network based on a slotted ALOHA protocol, vehicles may exchange information with surrounding vehicles, which typically includes event-driven Decentralized Environmental Notification Messages (DENMs), and periodic Cooperative Awareness Messages (CAMs), etc. In an environment congested with nodes, if a node has a high priority event-driven DENM to be broadcast and there is no vacant time slot to broadcast the message, safety problem may be caused. To solve this problem, a preemption mechanism is proposed by H. Agustin Cozzetti et al in “Scalability and QoS in MS-Aloha VANETs: Forced Slot Re-Use Versus Pre-Emption”. In this solution, when node A needs to broadcast a high priority message in a channel, it analyzes the channel for a whole frame period first. If all time slots are occupied, then node A scans Frame Information fields (FIs) in the frame to select a sub-frame with lower priority which is transmitted in a time slot, and preempts the time slot.